What Week of Pregnancy Am I In? How to Calculate

To find what week of pregnancy you are in, count the number of days from the first day of your last menstrual period (LMP) to today, then divide by 7. Pregnancy is measured in weeks from the LMP — not from the date of conception — which is why gestational age starts two weeks before a fertilized egg even exists.

This counting method matters more than most people realize. Every prenatal screening, every milestone, and every due date calculation your doctor uses is anchored to your LMP date. According to the American College of Obstetricians and Gynecologists (ACOG), gestational age based on LMP is the standard clinical reference throughout pregnancy. Knowing your exact week determines when to schedule your first ultrasound, when your anatomy scan is due, and when your pregnancy crosses into each trimester.

Use the free Pregnancy Week Calculator at RoughTools to find your current week and due date instantly — or follow the step-by-step method below.

The Gestational Age Formula

Gestational age — the clinical term for how many weeks pregnant you are — is calculated using two inputs: your LMP date and today's date.

Weeks pregnant formula:

Gestational Age (weeks) = (Today's date − LMP date) ÷ 7

Due date formula (Naegele's Rule):

Due date = LMP date + 280 days

Where:

LMP date — the first day of your last menstrual period before conception
Today's date — the current date
280 days — the standard full-term pregnancy duration (40 weeks × 7 days)
Gestational age — expressed in weeks and days (e.g., "12 weeks and 4 days")

Worked example: LMP of March 15, 2026, checked on June 13, 2026

Step 1 — Count days from LMP to today:

March 15 to June 13 = 90 days

March 15 → March 31: 16 days
April: 30 days
May: 31 days
June 1 → June 13: 13 days
Total: 16 + 30 + 31 + 13 = 90 days

Step 2 — Convert days to weeks and days:

90 ÷ 7 = 12 weeks and 6 days

Step 3 — Calculate due date (LMP + 280 days):

March 15 + 280 days = December 20, 2026

The result: on June 13, this pregnancy is 12 weeks and 6 days along — just one day before the end of the first trimester. The baby is due December 20, 2026. In clinical records, a doctor would write this as "12+6" (weeks + days).

In practice, your OB may adjust gestational age by 1–5 days after the first ultrasound if the measured crown-rump length suggests a slightly different conception date. The LMP-based calculation is the starting estimate; ultrasound dating at 8–12 weeks is the most accurate confirmation.

How to Calculate What Week of Pregnancy You Are In Step by Step

Find the first day of your last menstrual period. This is not the day your period ended — it is the first day it began. Check your phone's calendar, a period tracking app, or any notes from that time. If you genuinely do not know your LMP date, an early ultrasound (before 13 weeks) can determine gestational age within 3–5 days of accuracy.
Count the total number of days from LMP to today. Use a date calculator or count manually by noting how many days remain in the LMP month, then add complete months in between, then add the days elapsed so far in the current month. Count the LMP start date as Day 0 — today is the last day counted.
Divide the total days by 7. The whole number is your completed weeks; the remainder is the additional days. 90 ÷ 7 = 12 remainder 6, meaning 12 weeks and 6 days. Clinicians write this as 12+6.
Add 280 days to your LMP to get your estimated due date. This is the standard 40-week calculation. Most pregnancies deliver between 37 and 42 weeks — the due date is the statistical midpoint, not a guaranteed delivery date. Only about 4–5% of babies are born on their exact due date.
Identify which trimester you are in. The first trimester runs through the end of Week 13. The second trimester spans Weeks 14–27. The third trimester begins at Week 28 and runs through delivery. At 12 weeks and 6 days, you are in the final week of the first trimester.
Verify the result against your first appointment. If your OB has already confirmed your due date or gestational age at an appointment, compare that figure to your manual calculation. A difference of 1–7 days is normal and reflects rounding in clinical practice. A difference larger than 7–10 days suggests the LMP date used may be off — or that your OB is using ultrasound dating instead.

Pro tip: Save your LMP date somewhere permanent — in a notes app, a spreadsheet, or on paper. You will be asked for it at virtually every prenatal appointment throughout your pregnancy, and calculating gestational age from it becomes necessary any time you need to check on a milestone, screening window, or medication safety timeline.

What Week Does Each Trimester Start?

The first trimester starts at Week 1 (the LMP date itself) and ends at the end of Week 13. The second trimester runs from Week 14 through Week 27. The third trimester begins at Week 28 and continues until delivery.

| Trimester | Weeks | Key milestones | |---|---|---| | First | Weeks 1–13 | Heartbeat (~Week 6), nuchal scan (~Week 12), miscarriage risk drops sharply | | Second | Weeks 14–27 | Anatomy scan (~Week 20), movement felt (~Week 18–22), viability threshold (Week 24) | | Third | Weeks 28–40+ | Lung development, Group B strep test (~Week 36), full term (Week 37) |

The trimester boundaries matter because each one brings different screening tests with specific windows. The nuchal translucency ultrasound must be done between 11 weeks 0 days and 13 weeks 6 days — missing this window means the screening is no longer available. The anatomy scan is typically scheduled between weeks 18 and 22. Knowing what week of pregnancy you are in determines whether these windows are still open.

One thing most pregnancy trackers do not clearly state: "full term" changed in 2013. ACOG redefined full term as 39–40 weeks (previously 37 weeks). Babies born at 37–38 weeks are now classified as "early term," not full term, as outcomes differ meaningfully in that two-week window.

What Is the Difference Between Gestational Age and Fetal Age?

Gestational age counts from your LMP date; fetal age counts from the actual date of conception. They are typically 14 days apart — and gestational age is the standard used in all medical contexts.

The confusion is common. When your doctor says you are "8 weeks pregnant," the baby is actually closer to 6 weeks old as a developing embryo. Gestational age adds approximately two weeks to fetal age because ovulation and fertilization occur roughly two weeks after the LMP — but since the LMP is known and the conception date rarely is, medicine standardized on LMP dating.

This distinction matters when interpreting ultrasound measurements. If an 8-week ultrasound shows a crown-rump length consistent with 7 weeks 4 days, the sonographer will note the measurement but may not recommend changing your due date unless the discrepancy exceeds 7 days before 14 weeks (or 10–14 days later in pregnancy). Small measurement variations are normal and do not indicate a problem.

When tracking pregnancy milestones from apps or books, always confirm whether they reference gestational age (from LMP) or fetal age (from conception). Most reputable pregnancy resources — including ACOG guidelines and the due date calculator — use gestational age.

What Are the Key Pregnancy Milestones by Week?

The most clinically significant pregnancy milestones fall at weeks 6, 12, 20, 24, 28, 37, and 40. Each marks a transition in development, a required screening window, or a change in medical risk classification.

Week 6: Fetal heartbeat typically detectable by transvaginal ultrasound
Week 8–10: First prenatal appointment for most providers; first blood work panel
Weeks 11–13: Nuchal translucency ultrasound and first-trimester genetic screening (must be within this window)
Week 12–13: End of first trimester; miscarriage risk drops from approximately 10–15% to under 1%
Weeks 18–22: Anatomy scan — the most detailed ultrasound of the pregnancy
Week 24: Threshold of viability; survival outside the womb becomes medically possible
Week 28: Third trimester begins; glucose tolerance test typically done around this week
Week 35–36: Group B streptococcus (GBS) screening swab
Week 37: Early term begins; lungs are typically mature
Week 39–40: Full term; lowest risk of complications associated with timing of delivery

Knowing exactly what week of pregnancy you are in tells you which of these milestones are upcoming and which windows are still open. Use the pregnancy week calculator to see your milestones laid out against your specific due date.

Common Mistakes to Avoid When Calculating Your Pregnancy Week

Using the last day your period ended instead of the first day it started. The LMP is always the first day of bleeding — Day 1. Using the last day of your period adds 3–7 extra days to the calculation, which shifts your gestational age and due date forward by nearly a week. If your period typically lasts 5 days and you used Day 5, your due date will be approximately 5 days too late.
Counting from the date of intercourse or suspected conception. Conception typically happens 12–14 days after the LMP, but gestational age is not calculated from conception. Using a conception date understates your gestational age by approximately two weeks and produces a due date that is 14 days too late compared to what your OB will calculate.
Assuming the due date is exact. The due date is an estimate based on a statistical average. Normal full-term delivery can occur anywhere from 37 to 42 weeks. About 80% of babies are born within two weeks of the due date — in either direction. Planning travel, work leave, or logistics based on the exact due date without allowing a 2–3 week buffer is the most common practical mistake.
Not accounting for irregular cycles. Naegele's Rule assumes a 28-day menstrual cycle with ovulation on Day 14. If your cycles are consistently longer (say, 35 days), ovulation occurs around Day 21, and your actual conception was approximately 7 days later than the formula assumes. In this case, an early ultrasound is more reliable than LMP-based dating, and your OB may adjust your due date by up to a week.
Recalculating gestational age from a different LMP after ultrasound adjustment. If your doctor moves your due date after an early ultrasound, they are not changing your LMP — they are overriding the LMP-based estimate with a more accurate measurement. Do not recalculate from a new LMP to match the adjusted due date; use the ultrasound-confirmed due date directly.

Frequently Asked Questions

How do I know what week of pregnancy I am in without knowing my LMP? If you do not know your LMP, an early ultrasound between 6 and 13 weeks is the most accurate way to determine gestational age. The sonographer measures the crown-rump length (CRL) — the length of the embryo from head to bottom — and matches it to a standard growth table to estimate gestational age within 3–5 days. After 13 weeks, accuracy decreases slightly because fetal growth varies more between individuals.

What if my period is irregular — does the formula still work? The LMP formula is less reliable with irregular cycles because it assumes ovulation occurred on Day 14 of a 28-day cycle. If your cycles are 35–40 days, ovulation typically occurs later, which means conception happened later than the formula assumes — and the LMP-based due date will be earlier than the actual due date. If your cycles vary significantly, request an early dating ultrasound before 13 weeks for a more accurate gestational age.

What is the difference between gestational age and due date? Gestational age is how many weeks pregnant you are right now; due date is the projected end date of the pregnancy at 40 weeks. They are linked — knowing one gives you the other. At 12 weeks and 6 days gestational age with an LMP of March 15, the due date is December 20. As gestational age increases by one week, the due date does not change — it was set at the start of the pregnancy.

How many weeks pregnant is 3 months? Three months of pregnancy is approximately 13 weeks gestational age — the end of the first trimester. Pregnancy does not divide evenly into calendar months because months vary in length, which is why gestational age is always expressed in weeks. The common shorthand is: first trimester = months 1–3, second trimester = months 4–6, third trimester = months 7–9. But for any medical question, always use weeks and days, not months.

When should I use the pregnancy week calculator vs just asking my doctor? Use the pregnancy week calculator any time you want to quickly check your current gestational age between appointments, calculate how far along you will be at a future date (a vacation, a work event, a family gathering), or identify which screening windows are coming up. Your doctor is the right resource for interpreting ultrasound measurements, adjusting your due date, or answering clinical questions about development. Always confirm your gestational age and due date with your healthcare provider — the calculator provides an estimate based on LMP, not a clinical assessment.

Use the Free Pregnancy Week Calculator

The Free Pregnancy Week Calculator at RoughTools calculates your current gestational age in weeks and days, your estimated due date using Naegele's Rule, and which trimester you are in — all from a single date input. It also displays your upcoming milestone windows so you know exactly when to schedule your next screening. No account needed, no data stored, completely free.

Free Pregnancy Week Calculator →

You might also need:

Due Date Calculator — calculate your due date from LMP or conception date
Conception Date Calculator — estimate when conception occurred based on your due date
Pregnancy Weight Gain Calculator — track healthy weight gain targets by trimester
Ovulation Calculator — find your fertile window for future cycle planning